So, for the Samsung messages app:
It is not listed on Google Play or Samsung store., but Samsung messages app is installed by default. You probably did not see it because it is not on the home screen, but in Samsung folder in the Apps screen. If you change the default messaging app to Samsung messages, the app on the home screen will continue to be Google’s messages app. To open the Samsung one, you need to open the apps screen touch the Samsung folder and open the messages app there.
I have taken three screenshots. The first one is the home screen, of the icons at the bottom, that message app is Google’s. If you set Samsung’s to be the default one, that icon will open Google messages and will not work.
If you made Samsung messages the default app, then to open it. Open the Apps screen, by swapping up on the screen. You will get a list of apps and a search bar on top. Type “messages” on search bar and you will see both messages apps, Samsung’s and Google’s. See the second screenshot below for Samsung messages app, circled in blue.
If you want to have the Samsung messages app on the home screen, after searching for the app as described above, touch the Samsung app until the pop-up menu appears and select “Add to Home”. An icon for Samsung messages app will be created on the home screen and there is no need to search for it again. See the third screenshot below with the menu.
Just to add: if you want to put the Samsung messages app in place of the Google one on the icons on the home screen. You could touch the Google app icon as in the first screenshot until the pop-up menu appears and the click remove. Then if you created a shortcut of the Samsung app on the home screen, you can touch it and move it to the place where the Google app was.
